Metals are discharged into the air, water, and soil from natural sources such as continental dust, sea spray, biological activity, forest fires, and volcanic eruptions. In the last century, however, increased industrialization through mining, smelting, metal refining, production of metallic products and burning fossil fuels has released vast quantities of metals into the environment. These metals include mercury, cadmium, lead, zinc, silver, and tin from minerals, ores and bedrock. Because metals are non-degradable with infinite lifetimes, they become concentrated in food chains to toxic levels, placing a heavy burden on ecosystems. Besides the metal pollution toxicity problem, mineral and ore reserves are limited. Metal recovery processes allow conservation of scarce metal resources while decreasing environmental metal pollution.
Copper is used in the electrical industry, construction, industrial machinery, military supplies, and electroplating (Jolly & Edelstein (1988) in Copper (Jolly & Edelstein, eds.), U. S. Government Printing Office, Washington, D.C.). In 1987, the world production of primary (new) copper through mining was approximately 8.4 megatons (Mt)(Jolly & Edelstein (1988) supra; Crowson (1988) Minerals Handbook 1988-1989, Stockton Press, New York). The world reserve base (resources that are economically viable) is estimated to be 570 Mt, and the static reserve life of copper is 40 years. Obviously recycling of copper is important. The United States used 1.2 Mt of copper scrap in 1987, 41% of the total U.S. copper consumption. Besides conserving copper stores, recycling scrap copper only requires 3-40% of the energy required to extract pure copper from ore. Recycling scrap copper saves energy, conserves copper stores, and prevents further pollution of the environment from mining waste streams resulting from the smelting and refining processes.
Current methods for the removal and disposal of metals include diafiltration, adsorption on activated carbon, precipitation, ion exchange and encapsulation (Conner (1990) Chemical Fixation and Solidification of Hazardous Wastes, Van Nostrand Reinhold, New York). These methods are used as pretreatments to concentrate and fix metals before solidification. Before metals can be recycled from waste streams, the process must become economically feasible.
Since 1972, when Fujishima and Honda (Fujishima & Honda (1972) Nature 238:37) discovered that water could be decomposed into oxygen and hydrogen by illuminating titanium dioxide (TiO.sub.2), semiconductor electrochemistry has been studied for a variety of processes related to solar energy utilization (Bard (1979) J. Photothem. 10:59; Pruden & Ollis (1983) J. Catal. 82:404; Schaife (1980) Solar Energy 25:41; Kamat & Dimitrijevic (1990) Solar Energy 44:83). Semiconductors have been used in the photodeposition of metals such as gold, silver, platinum, palladium, rhodium, mercury, lead, manganese, uranium, thallium and cobalt from aqueous solutions (Serpone et al. (1988) in Photoreduction and Photodegradation of Inorganic Pollutants II. Selective Reduction and Recovery of Au, Pt, Pd, Rh, Hq, and Pb. (Serpone, Borarello & Pelizzetti, eds.), Kluwer Academic Publishers, Dordrecht; pp. 527-565; Amadelli et al. (1991) J. Chem. Soc. Faraday Trans. 87:3267; Ollis et al. (1991) Environ. Sci. Technol. 25:1523; Herrmann et al. (1988) J. Catal. 113:72; Tanaka et al. (1986) Solar Energy 36:159). Although numerous examples of metal photodeposition on semiconductors exists, the extent of removal and potential use under various solution conditions has not been thoroughly studied.
Particulate semiconductors are often used in photoelectrochemical experiments due to their low cost and large surface areas (7-50 m.sup.2 /g for TiO.sub.2). Illumination of TiO.sub.2 with a photon energy greater than the band gap (3.23 ev=384 nm for pure anatase) (Augustynski (1988) Structure and Bonding 69:1) excites an electron from the valence band to the conduction band, leaving a positively charged hole in the valence band. The conduction band edge (approximately -0.20 eV vs normal hydrogen electrode (NHE) at pH=0) represents the reducing power of the photogenerated electron, and the valence band edge (approximately 3.0 eV vs NBE) represents the oxidizing power of the photogenerated hole. Depending on solution conditions, the electron may reduce protons, water, dioxygen or metal ions. In a semiconductor particle, both the hole and the electron must be consumed to maintain neutrality. The hole may oxidize water to oxygen, oxidize water to hydroxyl radicals that in turn oxidize organics, or oxidize organics directly (Matthews (1988) J. Catal. 111:264). Theoretically, a solution species with a standard reduction potential positive of the conduction band edge and negative of the valence band edge can be reduced or oxidized by the electron or hole, respectively.
The photodecomposition of most metals involves the reduction of metal ions by the conduction band electrons and the oxidation of water to molecular oxygen by the valence band holes (Setpone et al. (1988) supra). Since many waste streams contain organics in addition to metals, the valence band holes could be used to oxidize organics instead. There are numerous examples of illuminated TiO.sub.2 used as a photocatalyst in the decomposition of a variety of organic compounds (Ollis et al. (1991) supra; Matthews (1988) supra; D'Olliveira et al. (1990) Environ. Sci. Technol. 24:990).
Reiche et al. (1979) J. Phys. Chem. 83:2248, studied the photoreduction of Cu(II) at TIO.sub.2. They observed the full reduction of Cu(II) to copper metal in aqueous solutions containing either acetate or no organic. Bideau et al. (1990) Chem. Eng. Comm. 93:167, studied the kinetics of the oxidation of formate in the presence of Cu(II) ions. They observed the formation of a red Cu-TiO.sub.2 species when solutions containing Cu(II) and formate were illuminated in the presence of TiO.sub.2. Morishita (1992) Chem. Lett. 10:1979, studied the photodeposition of copper onto TiO.sub.2 in aqueous solutions containing ethylenediaminetetraacetic acid (EDTA) or triethanolamine (TEA) at pH 5-13. Morishita observed that copper was reduced to either Cu (reddish black TiO.sub.2) or Cu.sub.2 O (black yellow), depending upon the solution conditions.
Cooper et al. (1990) Elucidation of Protocatalytic Purification Processes for the Removal of Trichloroethylene and Metal Ions from Water at Superfund Sites, USEPA Final Report No. 68D80059, studied the effect of metals in solution on the photocatalytic decomposition of trichloroethylene (TCE) under aerobic conditions. They found that the rate of TCE oxidation was dependent upon the type of metal present, its concentration, and the concentration of the organic. For example, in aqueous solutions containing 100 ppm TCE, Cu(II) concentrations of 10 ppm inhibited TCE destruction, but at a Cu(II) concentration of 1 ppm, TCE oxidation was not inhibited; in solutions containing 10 ppm TCE, 1 ppm Cu(II) was enough to slow the oxidation rate of TCE.
Nothing in the prior art discloses or suggests the removal of metal ions from solution using the reversible photoreductive process in combination with the resolubilization of the metal and regeneration of the photocatalyst as in the present invention. Cooper et al. (U.S. Pat. No. 5,174,877) describe a process for photochemical degradation of aqueous organic impurities using a variety of metallic containing catalysts, including TiO.sub.2. Cooper uses a cross flow filter for separating the fluid from the catalyst after the decomposition reaction is ended. Copper et al. (U.S. Pat. No. 5,118,422) describes an ultraviolet driven photocatalytic post-treatment technique for the purification of waste water distillates. Neither the Cooper patent discloses or suggests the cyclic process of the present invention, whereby separation of the catalyst and concentration of the metal ion is achieved by separation and oxidation. Bard et al. (U.S. Pat. No. 4,264,421) is directed towards the photodeposition of a variety of metals on TiO.sub.2 powder as a means of removing dilute concentrations of such metals from aqueous effluent. In contrast to the present invention, Bard does not attempt to reoxidize the metal and recover the TiO.sub.2 catalyst.